Typically, growth of Group-III nitride crystal is carried out by vapor-phase methods such as sublimation, or by liquid-phase methods such as flux growth. Growing by sublimation AlN single crystal, one example of a III nitride crystal, requires a crystal-growth temperature of over 1900° C., as described, for example, in U.S. Pat. Nos. 5,858,086, 6,001,748, and 6,296,956. Owing to the high temperatures sublimation requires, for the crystal-growth vessels employed, carbon components of materials such as graphite and pyrolytic graphite, and high-melting-temperature metals such as tungsten (W) and tantalum (Ta) are routine.
A problem associated with using carbon components for the crystal-growth vessels, however, is that carbon (C) atoms in the carbon components mix as impurities into the AlN single crystal, degrading its crystallinity. Meanwhile, problems with utilizing high-melting-temperature metals for the crystal-growth vessels are that, owing principally to nitridation and carbonization of the metals, high-melting-temperature metals as crystal-growth vessel components wear away, and that, due to quantitative changes in materials for the crystal-growth vessels, reproducibility in the growth of single-crystals is compromised.
Moreover, with vapor-phase methods including sublimation, the atomic ratio of the Group-III element (Al) to nitrogen (N) in the precursors (the source gases for growing III nitride single crystal, ditto hereinafter) inside the crystal-growth vessel, as well as the concentration of impurity gases therein, has to be controlled so as to be constant. Consequently, it has been necessary to form a special exhaust structure in the crystal growth vessels for discharging a portion of the precursors and impurity gases inside the crystal-growth chamber to the crystal-growth vessel exterior.    Patent Reference 1: U.S. Pat. No. 5,858,086    Patent Reference 2: U.S. Pat. No. 6,001,748    Patent Reference 3: U.S. Pat. No. 6,296,956